In a small boat or the like to which an outboard engine is attached, trolling so as to silently sail without generating large waves near a bay shore or a fishing place is often executed.
FIG. 7 shows a conventional arrangement of a control for the engine. In the conventional arrangement of FIG. 7, an output signal of a manual change-over switch 10 which selects ignition timing in the trolling mode, an output signal of a throttle sensor 30 which represents a throttle opening degree or position, and an output signal of a neutral switch 40 are input to an ignition signal output circuit 100. An output of the ignition signal output circuit 100 is transferred though an ignition circuit 50 to ignition coils 60 to 90 of the engine.
In this case, upon starting of the engine, to improve the starting efficiency, as shown in FIG. 8, ignition is performed at a timing IT.sub.6 at a throttle opening position .theta..sub.2. When the engine is set to the idling mode, that is, the throttle is substantially fully closed and the engine is not engaged with any drive gear, the ignition is executed at one of the ignition timings which has previously been selected by the manual change-over switch 10, that is, timings IT.sub.1 to IT.sub.5. The engine rotating speeds are set in accordance with the order such that IT.sub.1 &lt;IT.sub.2 &lt;IT.sub.3 &lt;IT.sub.4 &lt;IT.sub.5. When the drive gear to transfer the rotating force of the engine to the propeller is engaged, thereby initiating the trolling mode, the trolling sailing is executed at the engine rotating speed corresponding to the idling mode. Further, when the throttle opening degree is increased, as shown in FIG. 8, the ignition timing advances along a generally straight line. The ignition timing is saturated at a predetermined value, for instance IT.sub.7, and becomes constant.
Since the load in the idling (i.e. neutral) mode, where the drive gear is disengaged, is smaller than that in the trolling mode, where the drive gear is engaged, the engine rotating speed in the idling mode is larger than that in the trolling mode. Therefore, during the transition from the idling mode to the trolling mode (when the gear is engaged), often a dog clutch is damaged or the shifting operation becomes impossible.
On the other hand, to prevent such an inconvenience, it is sufficient to reduce the engine rotating speed in the idling mode. However, in this case, there is a drawback in that when the gear is connected, the engine rotating speed becomes too low and engine stall occurs.
It is an object of the present invention to avoid such inconveniences of the conventional arrangement and to provide an idling rotating speed control system for an outboard engine in which the shock is small, particularly when shifting from the idling mode to the trolling mode, and in which the engine rotation is stable.